I have not seen the relative humidity(RH) nor the parts per million (ppm) units in the drop down menu. of the points properties.
Point property units
ppm is a valid unit. You can type it in. I do see that it is not in the drop down, so thank you for bringing that to our attention.
"RH" doesn't seem to have a consensus as the unit of relative humidity. The internet suggests to me it is percent (%) or percent relative humidity, (% RH, I find this on Wikipedia, but also just %). This is probably why it was not included in that library of units (but % is). So, you can use % or you can change your Text Renderer on the data point to not use the unit as the suffix, and the instead supply the suffix you would like displayed, be it "RH", " RH" or "% RH"